4-NITRO-L-HISTIDINE AS A SUBSTRATE FOR HISTIDINE AMMONIA-LYASE - ROLE OF BETA-HYDROGEN ACIDITY IN THE RATE-LIMITING STEP

The Km for the interaction of 4-nitro-L-histidine with histidine ammonia-lyase (reduced enzyme, pH 8.0) is comparable to that for L-histidine, while Vmax is 1 8 that for the natural substrate. With the analog, addition of Cd+2 effects a small decrease in Km but fails to alter Vmax; the normal deuterium isotope effect for removal of the β-hydrogen (1.5-2.0) is eliminated; and enzyme-catalyzed incorporation of solvent tritium into substrate occurs to a much greater extent than into histidine. Thus, the nitro group increases the acidity of the β-hydrogen and the stability of the conjugate carbanion to such a degree that CH bond cleavage now precedes rate-limiting CN bond cleavage. © 1979.